In one type of prior accelerometer, a proof mass is mounted to a housing by a flexure hinge, and a force transducer is connected along the accelerometer's sensitive axis between the proof mass and the housing. An acceleration along the sensitive axis results in a compression or tension force on the force transducer. This force is converted into an electrical signal that indicates both the direction and magnitude of the acceleration.
In an accelerometer of the type described above, the coefficient of thermal expansion of the force transducer in general cannot be precisely matched by the coefficient of thermal expansion of the proof mass and housing. As a result, the proof mass moves relative to the housing as the temperature changes. This thermally induced movement has a number of adverse effects on the operation of the accelerometer. The flexure hinge resists the thermally induced movement and thereby causes a change in the bias of the instrument. A change in the axis alignment of the accelerometer also occurs as the thermally induced movement causes the position of the center of gravity of the proof mass to change relative to the housing. In addition, the thermally induced movement results in changes in the damping gap and the shock gap clearances between the proof mass and housing, thereby modifying the damping and limiting functions respectively of these components.
One method of providing temperature compensation for such an accelerometer is described in the U.S. patent application entitled Temperature Compensation of an Accelerometer, Ser. No. 879,262, Brian E. Norling, Inventor, filed concurrently herewith. This technique involves connecting two force transducers between the housing and the proof mass in such a way that differential thermal expansion or contraction results in rotation of the proof mass about a compensation axis normal to the sensitive axis. Such rotation is resisted by the flexure hinge, resulting in equal forces applied to both force transducers. The equal forces produce a common mode signal that can be eliminated by appropriate signal processing.
The twisting of a flexure hinge caused by thermally induced proof mass rotation produces certain side effects that may limit the usefulness of the described technique for certain applications. These side effects include a change in the compliance of the flexure to sensitive axis acceleration, and a resulting change in the accelerometer scale factor. Flexure twisting may also lead to a bias/temperature component in the accelerometer output. Axis alignment may also be influenced by flexure twisting. The reaction forces on the force transducers due to the flexure twisting may produce accelerometer output errors due to nonlinearities and inequalities in the force transducers. Finally, the torsional compliance of the flexure hinge may result in the flexure hinge being less rigid in one or both orthogonal axes, thereby reducing the mechanical natural frequencies of the accelerometer and limiting the accelerometer's g-range.